1. Field of the Invention
The present invention relates to a load test machine used particularly for a fatigue test to obtain a fatigue character of a test piece by applying a cyclic compression or tension load on the piece.
2. Related Art
Referring to the FIG. 10, a conventional load test machine will be explained. FIG. 10 is an illustration generally showing a conventional load test machine. The test machine has a base block 101 from which a pair of posts 103 are raised. Between the pair of the posts 103, there is a cross head 104 spanning between the posts 103. The cross head 104 is made of plates to define a hollow body. The posts 103 are jointed to the cross head 104 with bolts or the like. Furthermore, on the base block 101, there is mounted an actuator 111. Between the base block 101 and the cross head 104, a test piece 112 is removably secured to them with a pair of chucks 106. During a fatigue test, the actuator 111 applies a cyclic compression or tension load on the test piece 112. The frequency of the cyclic load is about 50 Hz.
Such a test machine is also used for obtaining a dynamic character of a rubber cushion to know an elastic constant and a damping coefficient thereof. During the test, a comparatively higher cyclic frequency is required in view of a practical use field of the rubber cushion. Thus, as disclosed in Japanese Patent Application Laid-open No. 57-48632, an air cushion is arranged between the hollow cross head and the posts to reduce the resonance frequency of the assembly to 0 (zero) to 5 Hz. An actual cyclic load frequency applied to the test is 10 to 500 Hz to obtain the elastic constant and the damping coefficient of the rubber cushion.
Conventional fatigue test machines have been generally used up to a material fatigue limit corresponding approximately to a 107 cycle load. However, recently, a 109 cycle load is required for obtaining a fatigue limit strength of a material used for providing a turbine construction member. It needs a load test machine allowing a higher frequency cyclic load to realize the 109 cycle load in a shorter period.
The conventional load test machine is generally used for a fatigue test with about 50 Hz. This frequency takes 56 hours to complete a fatigue test of a 107 cyclic load. But, it would take 232 days to complete a fatigue test of a 109 cycle load. Thus, an increased frequency for a fatigue test is desired to achieve a practical shorter test period. For example, if a fatigue test frequency is changed from 50 Hz to 1,000 Hz, a fatigue test of a 109 cycle load would complete in 12 days, remarkably improving its test efficiency.
The conventional load test machine has its resonance frequency higher than its test frequency, but it is practically difficult to increase the resonance frequency more than 200 Hz. To allow a 1,000 Hz frequency load, the conventional load test machine would require a constructional strength remarkably higher than a present one particularly concerning the posts 103 with a considerable increase in size and cost.
Meanwhile, a dynamic character measuring test machine uses a cyclic frequency load of 10 to 500 Hz (not of 50 Hz but up to 500 Hz) to know an elastic constant and a damping coefficient of a rubber cushion as described above. However, such test machines also are not constructed to enable a 1,000 Hz cyclic load. In the case of the conventional test machine disclosed in the laid-open application, an air cushion is provided between the posts and the cross head to achieve its lower resonance frequency of about 3 Hz. This allows a 10 to 500 Hz cyclic load but can not achieve a static load test (of about zero Hz). Furthermore, such an air cushion needs to be replaced to another one having another elasticity constant to change the resonance frequency thereof. This requires a desirable number of air cushions having elasticity constants different from each other to change the resonance frequency. Moreover, the air cushions have weights that are not easy in handling thereof, which takes an extra time to change the resonance frequency.